Unit 11 : Part 1 Reflection and Refraction of Light

Outline Wave Fronts and Rays Reflection Refraction Total Internal Reflection and Fiber Optics Dispersion

Wave Fronts and Rays Along a wave front, all the waves have the same phase. Rays are perpendicular to wave fronts, and indicate the direction of propagation of the wave. For a point source, the wave fronts are spherical and the rays radial.

Wave Fronts and Rays For a plane wave, the wave fronts are planes. Far from a point source, the waves are approximately planar.

Wave Fronts and Rays We see objects because rays from them, or appearing to be from them, enter our eyes.

Reflection The basic rule of reflection is that the angle of incidence equals the angle of reflection. Note that the angles are measured from the normal to the surface.

Reflection Specular reflection is reflection from a smooth surface.

Reflection Diffuse reflection is reflection from a rough surface.

Reflection Reflected rays are drawn using

Refraction Refraction occurs because the speed of light is different in different media. As light travels across a boundary between media, it bends. The relationship between the angle of incidence and the angle of refraction is:

Refraction Again, the angles are measured from the normal to the surface.

Refraction The index of refraction is the ratio of the speed of light in vacuum to the speed of light in the medium. The index of refraction is always equal to or greater than 1.

Refraction The frequency of the wave stays the same, but the wavelength of the light is shorter where the index of refraction is greater.

Refraction Snell’s law can be rewritten using the index of refraction: A ray bends toward the normal if the second medium has a larger index of refraction, and away from it if it is smaller.

Refraction A ray traversing a slab of glass will emerge headed in its original direction, but displaced sideways.

Refraction Mirages are formed by light refracting through air of different temperatures (and therefore densities).

Refraction Refraction can distort underwater views, make straight objects appear bent, and make submerged objects appear shallower than they really are.

Refraction Refraction through the atmosphere can make the Sun appear flattened at sunset, and increase the length of the day.

Total Internal Reflection and Fiber Optics When entering a medium of lower index of refraction, the refracted ray bends away from the normal. If the incident angle is large enough, the angle of refraction is 90°—no light escapes.

Total Internal Reflection and Fiber Optics The angle for which this occurs is called the critical angle: Light impinging on the boundary at this or a larger angle will be reflected; this is called total internal reflection.

Total Internal Reflection and Fiber Optics Total internal reflection prisms are used in binoculars to keep the length short.

Total Internal Reflection and Fiber Optics The index of refraction of diamond is particularly large; the multiple internal reflections contribute to the diamond’s brilliance.

Total Internal Reflection and Fiber Optics Fiber optics use total internal reflection to guide light along the fiber. The reflection means that there are no losses out the sides of the fiber.

Total Internal Reflection and Fiber Optics Fiber optic array

Dispersion Dispersion occurs because the index of refraction depends slightly on wavelength.

Dispersion Dispersion of sunlight in small drops of water produces a rainbow.

Summary The angle of incidence is equal to the angle of reflection. Index of refraction: Snell’s law for the refraction of light: Total internal reflection takes place for angles larger than the critical angle:

Summary Dispersion is the variation of the index of refraction with wavelength. It is the source of rainbows.